a. Field of the Invention
The instant invention relates to robotically controlled devices employing positional feedback systems. In particular, the instant invention relates to a method for obtaining a transfer function in order to calibrate actuation of a robotically controlled device relative to the positional feedback system.
b. Background Art
Catheters are used for an ever growing number of medical procedures. To name just a few examples, catheters are used for diagnostic, therapeutic, and ablative procedures. Typically, the physician manipulates the catheter through the patient's vasculature to the intended site, such as a site within the patient's heart. The catheter typically carries one or more electrodes or other diagnostic or therapeutic devices, which may be used for ablation, diagnosis, cardiac mapping, or the like.
It is well known that, to facilitate manipulation of the catheter through the patient's vasculature to the intended site, portions of the catheter shaft, especially the distal regions thereof, may be made steerable. For example, the catheter may be manufactured such that the physician can translate, rotate, and deflect the distal end of the catheter as necessary and desired to negotiate the tortuous paths of the patient's vasculature en route to the target site.
By way of illustration, deflectability is oftentimes achieved by installing one or more steering wires (sometimes referred to as “pull wires”) along the length of the catheter shaft. These steering wires are coupled to one or more actuators that the physician can utilize to selectively tension the wires, thereby deflecting the distal end of the catheter. It is also known that the pull wires may be coupled to a motorized, electromechanical control system for actuating the catheter on the deflection axis. Similarly, in order to advance and retract (that is, translate) the catheter, the catheter may be coupled to a motorized carriage.
Positional feedback systems (sometimes referred to as localization systems, navigation systems, or mapping systems, with the various terms being used interchangeably herein) may be used to provide the physician with information concerning the position or location of the catheter within the patient. U.S. Pat. No. 5,697,377 (“the '377 patent) and U.S. Pat. No. 5,983,126 (“the '126 patent”), both of which are hereby expressly incorporated by reference as if fully set forth herein, disclose navigation systems for determining the position or location of a catheter in a patient's heart.
In the systems of the '377 and '126 patents, current pulses are applied to pairs of orthogonally-arranged patch electrodes placed on the body of the patient. These patches are used to create electric fields inside the patient defining a set of orthogonal x, y, and z measurement axes. The patents teach small amplitude, low current pulses supplied continuously at three different frequencies, one on each axis. A location electrode placed within these electric fields—for example, within the patient's heart—experiences voltages depending on its location between the pairs of patch electrodes defining each axis. The voltage on the location electrode, when compared to that on a reference electrode, indicates the position of the location electrode relative to the reference electrode. Thus, the three voltages can be used to define a location of the location electrode, and thus the catheter, in three-dimensional space, which may be expressed as a rectangular (x, y, z) coordinate relative to a set of orthogonal measurement axes.
While the motors used to actuate a catheter are themselves quite precise, the mechanical systems employed to deflect, translate, or rotate the catheter are less so, especially where actuation forces must be transmitted over significant distances. In particular, the position of the catheter tip depends upon many variables, including the catheter's temperature, its recent movement history, and the tortuous path it is traversing, as well as the expected and desired dependence upon the displacement supplied to the pull wires or other mechanical and electromechanical system elements. Much of this variability is due to retained forces along the length of the catheter body and internal catheter structures, which may be collectively referred to as “memory.” In fact, for a given displacement of the pull wires, these factors can result in a variation of the tip location in excess of 1 cm. Relative changes desired in tip position are not precisely predictable for the same reasons.
Furthermore, extant positional feedback systems, such as the navigation system described above, may have inherent error. Though intra-cardiac navigation systems are robust in terms of their reproducibility, the dimensional feedback that they provide tends to be contextual—dependent upon the particular patient, heart chamber structure, and other factors. Though this presents no difficulty for mapping applications, wherein all sites are mapped and marked in the same relative context, it does present a problem in open loop characterization of the catheter. For example, if the navigation system indicates that a 10 mm deflection is necessary, but this movement is, in reality, only 9 mm per the catheter's characteristics, an error of 1 mm results. This navigation system error is in addition to the device error discussed above.
Thus, it is desirable to obtain a transfer function relating the desired motion of the catheter in three-dimensional space to the control vectors or motion commands (referred to herein as “movement vectors”) that are supplied to the motors. A first order calibration method might be to actuate the catheter for an expected movement and measure the actual movement utilizing the navigation system described above. A scale correction factor can be derived from the ratio of the expected movement to the actual movement. This approach, however, may account for some uncertainties of the catheter itself, but does not account for external error sources such as patient motion, cardiac motion, patient respiration, and electronic noise.